Bugfix: Make `check` command honor `RESTIC_CACHE_DIR` environment variable

Previously, the `check` command didn't honor the `RESTIC_CACHE_DIR` environment
variable, which caused problems in certain system/usage configurations. This
has now been fixed.

https://github.com/restic/restic/issues/3382
https://github.com/restic/restic/pull/3474
